If you add half a gram of something that dissolves completely your total dissolved solids have, obviously, gone up by half a gram. The catch here is that your TDS meter doesn't read TDS it reads conductivity. Meters are often, but not always, calibrated for sodium chloride so what you would have to do is add say 100 mg CaCl2 to a liter of DI water (giving 100 mg/L TDS) and read that with your meter to see how many mg/L TDS reading units you get per mg/L CaCl2.

Yes, my inexpensive TDS meter is calibrated to about 300 ppm NaCl. I found some documented conductivity values for CaCl2 in DI water but your experiment seems a lot more practical.

I'd probably want to do independent tests with my dosing rate of Campden, 100 mg/L CaCl2 & 100 mg/L of CaSO4 to accurately predict the TDS of brewing water and build the prediction into the water spreadsheets for a quick check. Probably one of those things you do once out of curiosity...

If you start with RO water the measures 22 ppm TDS (down from about 400 ppm TDS), and you add 0.5 g CaCl2 per gal of all brewing water, is it possible to calculate the brewing water TDS?

Yes, as A.J. pointed out, but what would that value tell you? TDS (conductivity based or weighing the residue after evaporating all water) doesn't tell you anything about the composition of the water. It's only useful if its a low number since you would be able to set the water profile by adding salts.

Yes, as A.J. pointed out, but what would that value tell you? TDS (conductivity based or weighing the residue after evaporating all water) doesn't tell you anything about the composition of the water. It's only useful if its a low number since you would be able to set the water profile by adding salts.

Kai

I agree it has no pratical use, mash pH is the reason I use RO water. I bought a TDS meter to know if the RO membranes are working well. It's not hard to measure out salts so measing the TDS as a check isn't necessary. It seemed like an easy problem to put to words and something I would have liked to know the math behind.

I agree. TDS meters are good for assessing the effectiveness of your RO system. DSmith pointed to the math: They have to be calibrated to one salt and you could try to find out which. If you want to measure salt additions you could also use a GH&KH test kit. But then again, you know how much salt you added

They don't necessarily have to be calibrated to a particular salt though they often are out of the box and that's typically sodium chloride. The point is that they measure conductivity which is a function of the concentration of each ion and its mobility. If you have constant or fairly constant ratios of ion concentrations you could calibrate to the mix. So if, for example
you had a Ward Labd report like this one (the first I could find looking back)

we have a reported TDS estimated by conductivity of 47 whereas the sum of the reported ions give us 60. The reasonably close balance (0.1 mEq/L) says this is a pretty good report and so we can conclude that

TDS ~ 60*conductivity(mmho/cm)/0.08

If, in the spring, there is snow melt and we see conductivity of 0.04 we are probably safe in assuming that the TDS is 30, that the alkalinity is about 15 etc. If, OTOH, the conductivity suddenly goes up to 0.1 in the winter we might suspect that it is due to chloride from road treatment and this crude approximation might not be so good.

If your meter only reads TDS, i.e. you don't get an actual conductivity reading then use

TDS ~ 60*TDS_reading/47

In place of 47 use whatever your meter read on this sample. IOW, check the TDS using your meter before sending the sample to Ward Labs and compare to the calculated TDS you get by summing all the reported ion contents (don't forget to multiply the Sulfate number by 3 and the Nitrate number by 4.43).